This project represents an attempt to understand the mechanism of freeze-induced damage to erythrocyte membranes. Freeze injury was simulated by exposing either red cells or ghosts to hypertonic solutions of sodium chloride from 295-3055 mOsm. Membrane changes were assessed with the aid of spin-labeled probe molecules. Hypertonic sodium chloride solutions produced no detectable changes in electron spin resonance spectra of red cells labeled with stearic acid spin labels. However, an androstane spin label did undergo a small decrease in mobility when red cells were exposed to 3055 mOsm sodium chloride. Ghost membranes labeled with either maleimide or iodoacetamide spin labels showed an increase in the population of highly immobilized labels as the concentration of sodium chloride was increased. These observations suggest that in hyperosmotic sodium chloride an increased rigidity of the red cell membrane develops which is probably due to a decrease in the fluidity of the phospholipids accompanied by conformational changes of the red cell membrane proteins.